In a multi-hop communication system, communication signals are transmitted along a communication path or link from a source apparatus to a destination apparatus via one or more intermediate apparatuses or nodes. FIG. 1a illustrates a single-cell two-hop wireless communication system comprising a base station BS 10, a relay station RS 30, and a mobile station MS 20, also referred to as user equipment (UE). Such a system, employing mobile stations as well as relay stations (fixed or mobile) in a multi-hop arrangement, is called a mobile multi-hop relay (MR) system. By including the RS 30 in the communications chain, the BS 10 can serve an MS outside its normal range, for example when the MS is located indoors where signals from the BS do not reach sufficiently. For simplicity, FIG. 1a indicates only the downlink, in other words a communications channel from the BS 10 via the RS 30 to the MS 20. In practice, however, an uplink (channel from the MS back to the BS) is normally present as well, though this need not necessarily be routed via the same RS 30. In some wireless communication systems, notably those using WiMAX technology (IEEE 802.16 and its variants), a link such as RS-MS terminating at an MS is called an access link, and in this case the RS is also referred to as an access RS. Meanwhile, the link BS-RS between the BS and RS is called a relay link.
It is possible for more than one RS 30 to be involved in the communications chain from BS 10 to MS 20 and in this case a relay link RS-RS is defined between each two RSs in the chain. Moreover, it is possible for more than one access link to be established simultaneously to the same MS 20, e.g. from different relay stations (this is called co-operative relay). Different communication paths may exist for user data (data useful to the user, such as a voice call or video stream), and control information (for signalling purposes and housekeeping in the system).
In practice, of course, several BSs, RSs and MSs are combined to form a larger system, and communication links between nodes (wireless transceiver units) in the system are continually formed and re-formed as user demands and locations (and thus the positions of MSs relative to the other nodes) change. Typically, multiple MSs are associated (or “attached”) to each RS (access RS) 30. In addition to data transmissions via the downlink and uplink, various control signals are exchanged between the nodes of the system. The BSs are normally in mutual communication via a wired backbone network in addition to being able to communicate wirelessly.
In FIG. 1a, the RS 30 is assumed to be fixed, as would be the case if installed in an office building for example, but FIG. 1b shows a variation of the FIG. 1a scheme in which the RS itself is mobile, for example installed in a vehicle. In this case, the RS is referred to as a Mobile Relay Station MRS 300. An MRS 300 installed in a bus, for example, may serve several MSs 20 which move along with the MRS. The BS may be relabeled as an MR-BS 100 to denote that it has the capability to support relay stations. In the remainder of this specification, the term BS is used synonymously with MR-BS. Also, the term RS includes MRS wherever appropriate; for example, an RS-RS link may be formed with an MRS at either or both ends. Mobile multi-hop relay can be summed up as the concept of relaying user data between an MS and MR-BS, and possibly control information between an MR-BS and an MS or RS, through one or more RSs. It may be possible to establish multiple communication paths between an MR-BS 100 and an MS 20, including possibly a direct path not involving an RS such as the MRS 300, and to communicate the same user data and/or control information though the multiple paths to improve performance.
Another issue in multi-hop systems is whether or not each MS is “relay aware”, in other words whether or not the MS knows that it is communicating with a RS rather than directly with the BS. Clearly, a “relay aware” system places greater demands on the mobile stations and is less likely to be compatible with legacy hardware. Variations of the 802.16 standard currently under development include 802.16j and 802.16m. In 802.16j, the MSs are not relay-aware, that is they are not aware of the presence of relay stations in the network. By contrast, in 802.16m the MS are assumed to know when they are communicating with an RS and to be able to adapt their behavior accordingly. The invention to be described is most readily applicable to such “relay aware” systems. As 802.16m is expected to use many of the same principles and techniques of 802.16j, reference will be made in the following description to various features already proposed for 802.16j.
In any IEEE802.16-based system, data is transmitted in units of frames divided into downlink (DL) and uplink (UL) sub-frames which can either be simultaneous (FDD) or consecutive (TDD or H-FDD). Each DL sub frame may start with a preamble followed by a Frame Control Header (FCH), and a DL-MAP and UL_MAP to indicate the subsequent frame structure. In addition, a downlink channel descriptor DCD and uplink channel descriptor UCD are periodically sent to indicate burst profiles (modulation and error-correction schemes) employed.
FIG. 2 illustrates a number of applications for relay stations. For fixed infrastructure, the coverage provided by a relay station may be “in-fill” to allow access to the communication network for mobile stations which may otherwise be in the shadow of other objects or otherwise unable to receive a signal of sufficient strength from the base station despite being within the normal range of the base station. This is shown in the top half of FIGS. 2a and 2b. “Range extension” is also shown, in which a relay station allows access when a mobile station is outside the normal data transmission range of a base station. One example of in-fill shown in the top half of FIG. 2b is positioning of a nomadic relay station to allow penetration of coverage within a building that could be above, at, or below ground level. Conversely, a relay station may be used on the uplink to allow low power transmissions from an MS to be picked up and passed on to a BS. In this way, the size of the cell served by the BS is effectively increased. The area of coverage of the BS and all of its associated relay stations is called an MR-cell.
Other applications are nomadic relay stations which are brought into effect for temporary cover, providing access during events or emergencies/disasters, as illustrated in FIG. 2a. Such a nomadic RS would normally be considered as fixed for control purposes, as it will not move over short timescales.
A final application shown in the lower half of FIG. 2b provides access to a network using a MRS positioned on a vehicle. Coverage is provided for MS devices which are travelling together on a mobile vehicle, such as a bus or a train. The MRS is mounted on the vehicle and it connects to an MR-BS or another RS via a mobile relay link. The MRS provides an access link to MS/SS devices riding on the platform. This is another instance of in-fill, because in the vehicle, each MS may be unable to receive a sufficient signal strength even if the vehicle is within the coverage range of an MR-BS or RS. The MRS is able to receive the signal from the MR-BS/RS and can communicate at the same time with the mobile stations in the vehicle.
As users' needs in such a network change, for example as they request new or additional services, the amounts of data being received and transmitted and hence their bandwidth (BW) requirements also change. In a system of the FIG. 1/FIG. 2 type, bandwidth requests may be directed first from an MS to the RS and from there to the MR-BS. Here, the requests are generally for uplink bandwidth since the MR-BS (or possibly, RS) will itself determine the allocations of bandwidth on the downlink, taking into account the services currently being provided to users.
For simplifying the discussion of bandwidth requests, it may be helpful to think of “subordinate” and “super-ordinate” stations or nodes in the system. Here, the term “super-ordinate” refers to any “higher” or “upstream” node (where “downstream” is normally thought of as the direction leading to the MS). Conversely, “subordinate” refers to a “lower” node, in other words a node “downstream” of another node. The MS or RS is referred to as a subordinate station of the MR-BS, which is also referred to as their super-ordinate station. The RS, in turn, is a super-ordinate station with respect to the MS. In a link of more than two hops, one RS will be a super-ordinate node of another RS.
In a wireless relay system, there are several ways for a subordinate station (MS or RS) to obtain bandwidth from a super-ordinate station (RS or MR-BS), and these can be summarized as follows:
Dedicated bandwidth: After the subordinate station is attached to the network initially in a network entry procedure, the system will allocate dedicated bandwidth to the station automatically. At this stage, there is no explicit bandwidth request. When dedicated bandwidth needs to be adjusted, there are two schemes to trigger bandwidth reallocation. One involves the MR-BS (or RS) monitoring other relevant messages from their subordinate stations, the other is for the MS (or RS) to request bandwidth explicitly by sending a bandwidth request header (see below). As for the former possibility, the system (more particularly, the super-ordinate MR-BS) extracts useful information from the relevant messages and reallocates the current dedicated bandwidth accordingly.
Bandwidth request header: With this 48-bit length header issued by a downstream node (e.g. MS), the super-ordinate station can know the amount of bandwidth requested by the downstream node, because there is an explicit bandwidth number (either incremental or aggregate) in the header. In some cases, the allocation period (time of validity of the allocation) is also included. In addition, the super-ordinate station can identify the connection (and hence the downstream node) for which bandwidth is requested, by a connection ID (CID) included in the bandwidth request header.
Bandwidth request message: Usually, the message's length is not fixed. In different cases, the message can carry different information. The system needs to monitor it and identify the bandwidth requested.
CDMA Bandwidth Request: The subordinate node needs to select one CDMA ranging code from a special CDMA ranging code subset and send this code in a special region in the uplink frame. When the super-ordinate node receives this code correctly, it should allocate a fixed small amount of bandwidth sufficient to provide a request opportunity allowing the subordinate node to send a bandwidth request header, for which see above. The subordinate node can transmit other messages/headers or data on this bandwidth depending on its decision as well.
The details of the BW request procedure will depend on whether or not the MS are relay-aware. For the moment, let us consider an 802.16j system, in which the mobile stations are not relay aware.
In an IEEE 802.16j system, there are two scheduling modes:
Distributed scheduling mode: A mode of operation applicable to multi-hop relay where the MR-BS and each RS in the MR-cell (with or without information from the MR-BS) determine the bandwidth allocations and generate the corresponding MAPs for the access link to/from their subordinate MSs and/or relay links to/from their subordinate RSs.
Centralized scheduling mode: A mode of operation applicable to multihop relay where an MR-BS determines the bandwidth allocations without active participation of each RS in the cell, and generates the corresponding MAPs (or dictates the information used by RSs to generate their MAPs) for all access and relay links in the MR-cell.
Relay stations can be classified as transparent or non-transparent depending on whether or not they are capable of transmitting DL frame-start preamble, FCH, MAP message(s) and channel descriptor (DCD/UCD) messages. A non-transparent RS can transmit this information, whereas a transparent RS does not. Consequently, a non-transparent RS can operate in both centralized and distributed scheduling mode, while a transparent RS can only operate in centralized scheduling mode.
In the distributed scheduling mode, on the relay link, the contention-based CDMA bandwidth request process and its associated ranging codes may be the same as those used on the access link. Alternatively, a modification may be introduced into distributed scheduling on the relay link to accelerate the bandwidth request/allocation procedure in an IEEE 802.16j system. The MR-BS may assign unique RS CDMA ranging codes to each RS in its MR-cell for the purpose of requesting bandwidth from a super-ordinate station. The RS may reduce the latency of relaying traffic by sending a bandwidth request CDMA ranging code as soon as it receives one from a downstream station instead of waiting for the actual packets of user data to arrive (see FIG. 3).
In an IEEE 802.16j system, as well as two different scheduling modes, two different security modes are defined. The centralized security mode is based on key management between MR-BS and MS. The distributed security mode incorporates key management between MR-BS and access-RS and access-RS and MS. Shared security information between subordinate and super-ordinate nodes forms a “Security Association” (SA) including Traffic Encryption Keys (TEKs).
There are some messages which can only be transmitted on the access uplink. For example, in the distributed scheduling mode of a IEEE 802.16j system, a so-called SA-TEK 3-way handshake procedure can take place between access RS and MS. In such a case, if it is necessary for an MS to ask for bandwidth by sending a CDMA ranging code, the access RS receiving a conventional CDMA code cannot determine whether the bandwidth which the MS requires is for a multi-hop link or a single hop link. In such a situation, the access RS can respond to the CDMA bandwidth request in one of the following ways:    (i) the access RS waits for the actual packets of data to arrive from MS, then decides to request uplink bandwidth from the super-ordinate node; or    (ii) the access RS sends a bandwidth request CDMA ranging code to the MR-BS before the actual packets from the MS start to arrive (as in FIG. 3 referred to above).
However, if the MS is asking for multi-hop link bandwidth, method (i) will lead to longer latency. On the other hand, if MS is asking for single hop link bandwidth, method (ii) will lead to an unnecessary bandwidth request being made, and to a wasteful bandwidth allocation on the uplink of the access RS (see FIG. 4). So, neither of these two methods meet the requirements of latency and bandwidth utility efficiency at the same time.